1. Field of the Invention
The present invention relates to a piezoelectric ceramic, and more particularly to a piezoelectric ceramic which is useful in a high frequency range of 10 MHz or more. The invention also relates to a piezoelectric oscillator using the piezoelectric ceramic and to a method for producing the piezoelectric ceramic.
2. Background Art
A piezoelectric ceramic including lead titanate as a main component has been often employed for a piezoelectric oscillator which has a vibration frequency of 10 MHz or more.
In such type of oscillator, the third order vibration mode of longitudinal vibration in the oscillator is utilized. The use of this mode is preferable especially in the high frequency region to achieve good response to an input signal. A method for enhancing the response of third order vibration of a piezoelectric ceramic in which pores of the ceramic are reduced so as to densify the ceramic is known. However, in addition to improving the response of third order vibration, densification of a piezoelectric ceramic also enhances the response of fifth order vibration, which is unnecessary when a third order vibration mode is used. Thus, such a piezoelectric oscillator generating fifth order vibration needs improvements in its characteristics.
There is a method for suppressing the response of fifth order vibration by forming pores dispersed in a piezoelectric ceramic. However, in this case, porosity of the ceramic increases to disadvantageously deteriorate the mechanical strength and reliability of the piezoelectric ceramic.
In view of the foregoing, the present invention provides a piezoelectric ceramic suppressing the response of fifth order vibration without deteriorating the mechanical strength and reliability thereof. In accordance with another aspect of the present invention, a method for producing the piezoelectric ceramic is provided. Still further, the present invention provides a piezoelectric oscillator using the piezoelectric ceramic.
Accordingly, in a first aspect of the present invention, there is provided a piezoelectric ceramic comprising lead titanate as a primary component, wherein the primary component contains a titanium oxide crystalline phase. By employment of such a constitution, the response of fifth order vibration decreases to thereby suppress unnecessary vibration without deteriorating the mechanical strength and reliability of the piezoelectric ceramic.
Preferably, the grain size of the titanium oxide crystalline phase is about 4 to 28 xcexcm.
In a second aspect of the present invention, there is provided a method for producing a piezoelectric ceramic comprising a step of firing a compact formed of a piezoelectric ceramic predominantly containing lead titanate at about 1230 to 1245xc2x0 C. By employment of such a temperature range, the response of fifth order vibration can be suppressed without substantial suppression of that of third order vibration.
In a third aspect of the present invention, there is provided a method for producing a piezoelectric ceramic comprising the following steps (1) to (5):
(1) mixing a piezoelectric ceramic material containing lead oxide and titanium oxide to thereby produce a mixture;
(2) firing the mixture to thereby produce a calcined product;
(3) crushing the calcined product and adding titanium oxide powder and a binder to the crushed product to thereby produce a binder-containing mixture;
(4) molding the binder-containing mixture to thereby produce a compact; and
(5) firing the compact. By employment of such steps, dispersion of a titanium oxide crystalline phase into lead titanate serving as a primary component is ensured.
In a fourth aspect of the present invention, there is provided a piezoelectric oscillator comprising an insulating substrate, a piezoelectric ceramic element, lead terminals and an outer resin covering for the piezoelectric ceramic element, wherein
the insulating substrate has patterned electrodes thereon;
the piezoelectric ceramic element comprises the piezoelectric ceramic and electrodes formed on main surfaces of the ceramic and electrically connected with the patterned electrodes; and
the lead terminals are electrically connected to the patterned electrodes.
In a fifth aspect of the present invention, there is provided a piezoelectric oscillator comprising a piezoelectric ceramic element, upper and lower substrates sandwiching the piezoelectric ceramic element, and external electrodes, wherein
the piezoelectric ceramic element comprises the piezoelectric ceramic and internal electrodes formed on main surfaces thereof, and
the external electrodes are electrically connected to the internal electrodes and formed on sides faces of the piezoelectric ceramic element and side faces of the upper and lower substrates.
By employment of such a constitution, there is provided a piezoelectric oscillator in which the response of an unnecessary fifth order vibration is suppressed.